Elongated joint implants are known, which can be inserted into tubular bones, in which the cross section decreases from the proximal to the distal end in order to adapt the shaft to the actuality of the bone or to reduce the amount of material to be removed when preparing the recess for receiving the implant.
To anchor the implant in the bone, on the one hand the use of bonding agents, for example on a plastic basis, has become known or, on the other, techniques operating without bonding agents for anchoring which, in case of direct anchoring, prevent problems caused by heat generation and incompatibility of the tissue with the bonding agent. However, the desired goal is to avoid a prolonged waiting period required for the implant to grow in after insertion of the shaft part and to obtain, immediately after the implant has been inserted, a stable connection between shaft and bone.
The recess in the bone for receiving the shaft of the implant is generally formed by driving a rasp, cutter or broaching tool which generally corresponds to the shape of the shaft, into the bone in the direction of the diaphysis. If the various known implant shafts, which mostly have curved shapes, adjoin the walls of the recess only in a few places and over relative short areas, the transfer of the mechanical forces occurs in these areas across zones with comparatively small surfaces and therefore with a high pressure load. However, such pressure spots cause a disruption of the biological balance inside the bone, to which it reacts in that in these areas of increased pressure stress a breakdown of the bone substance takes place and thus a weakening of the cortex and therefore the danger of a loosening of the seat of the shaft.
Placement of longitudinally extending ribs on the surface areas of the shafts does not remove this problem. Although the edges penetrate into the bone tissue, they laterally displace it so that this also causes increased pressure stresses which, because of the linear load on the bone, can burst it. Furthermore, the contact area between the bone and the shaft of the implant is not noticeably increased by this method; in such a case only the longitudinally extending ribs, not the surface area, adjoin the bone or penetrate it.